A few weeks ago I posted about this problem and thought I found it. Here's the background on my '65 352 (all stock):
1) Replaced timing chain and gears with Cloyes double roller
2) Though I know better, I left my brain in the house and forgot to install the dowel from the old cam gear into the new one. Better still, I started the engine, which ran!
3) However, when I started it the second time all want amiss and I had no compression, as everything was out of timing sequence. When this happened, I bent 1 pushrod severely, and may have bent 2 others slightly (they could possibly have been that way before). All bent pushrods were on the driver's side head.
4) Replaced pushrods, thinking this is what made the "rap" sound. It's still there.
5) The "rap": does not do it at idle, though when it first starts you can sort of detect it for a second before the engine settles into idle. Any application of throttle results in a metallic sound, but I cannot tell where it is coming from other than to say it is not a bottom end sound (I know what those sounds are). Head? Cam? Timing gear area? Distributor? Can't narrow it down.
Diagnostics:
A) I have removed all pushrods and checked them again.
B) Have used a surgical light to look into the cylinders. There are one or two half moon valve marks in 2 cylinders (driver's side, see above) that could possibly indicate valve problem. Consider that these marks could have been made when the pushrods got bent, and valves may be okay.
C) Turned engine by hand and examined valve train function. Nothing unusual.
D) Tried using a straight edge across top of driver's side valve stems to see if any were substantially shorter, indicating a bent valve stem. Nothing unusual.
E) Depressed valves manually to see if anything was hanging up (bent valve stem in a guide, for example). Nothing found.

Additional info: The Cloyes set box noted that something on the cam retainer should be ground off...however, my 352 did not seem to have this oblong bit, nor were there any clearance issues noted the 3 or 4 times I took it apart so far. I'm guessing this caution applies to some, but not all, FE's.

Since the noise has been constant ever since first startup (remember, the truck ran fine first time) I'm not convinced the problem is in the head. Distributor was not removed. Is there anything in the timing gears, fuel pump drive, etc that could possibly be causing an rpm related metallic "rap"? Aside from the worn timing chain, this is a 93,000 mile engine that shows incredible maintenance (very clean...virtually no sludge) which fits the rest of the truck.

 

Before you do anything, check for an exhaust leak. They can sound very much like a metallic tap - FEs are notorious for this. I find it useful to let the engine cool overnight to do this check: Have an assistant start the engine. Listen for the tap (caused by the exhaust leak) at startup, and start checking on that side. Feel around each each exhaust port, and have your assistant goose the engine at each port. Don't forget to check the pipe to manifold doughnut.
On the driver's side, don't forget to check the heat riser valve, especially on the back side.
If you can eliminate all evidence of exhaust leaks, then it is time to look at the internals of the engine. See my next post.

 

1. is it possible you damaged a lifter when the cam got off track?

2. yes, the fuel pumps have been known to make noise. try another pump if possible.

Does it sound like it's at both sides of the block? the fuel pump would be ticking against the cam eccentric which would probably sound like it's coming from everywhere...

 

I think one of your best investments now is a mechanic's stethescope. VERY valuable for finding these types of things.
Warm the engine up so it will idle well, and remove the valve cover. You will lose a little oil but not much - and possibly not lose any.
You may find it helpful to have an assistant goose the engine at each hearing point, to confirm your findings.
Check the following with the stethescope:
1. Check each rocker arm, all along the rocker arm rod. (Check between the springs, as close to each rocker arm as you can.)
2. Check the solid parts of the head, especially next to each valve.
3. Check the water pump, fuel pump, and the alternator for noises.
4. Check the head and the exhaust manifold near each exhaust port. Goose the engine at each one; sometimes a leaking port or doughnut at the pipe junction wil sound like a metallic tap.
5. Listen to the timing gear cover at the alternator side and the fuel pump side (if you can reach it.)
6. Check the distributor housing, both the shaft and the upper housing.

You will be looking for the tapping noise to get louder as you get closer to it.

 

Finally, if the stethescope doesn't help or gives you inconclusive results, you can try checking for collapsed lifters. The check involves looking for slop between the pushrod and the rocker arm.

The most accurate way is to check with the engine off. You will need to have each cylinder at TDC, and at the top of the compression stroke to get any useable results. With the cylinder at TDC of the compresssion stroke, push and pull on the rocker arm. There should be NO slop between the pushrod and the rocker arm (or between the rocker arm and the valve stem). If there is, you have found a problem.

A less accurate but faster check it to do it with the engine running. With the engine warmed up and idling with the valve covers off, push down on each rocker arm at the valve and at the pushrod end. If you feel a secondary bump or click, you have found slop, and found a problem. There should be no slop between the pushrod and rocker arm.

I prefer to do the running test first, then do the dry test to confirm what I think I found.

And yes, although it is a little tricky, you CAN change FE lifters without removing the intake manifold.

 

i ran into that problem before, i found out it was the fuel pump encentric hitting the timing cover. the new timing chain set-up i bought had a provision to remove a spacer between the upper timing chain sprocket and cam. just a thought, good luck

 

I agree about the exhaust..those donuts make a rapping sound when they go out and can be real loud.

 

to jump on the train i agree too ! they sound like your lifters are tapping away, and it happens all so sudden !

 

After giving this some more thought, I see some more (but more remote) possibilities:

1. Loose spark plug
2. Loose cam eccentric for the fuel pump
3. Wrist pin or piston slap problem

 

This was exactly the problem!
After pulling my hair out (and tearing the engine apart multiple times) Led Sled hit on it. I doubt I would have found it otherwise. I was ready to start pulling cylinder heads.
The Cloyes cam gear is made thicker, so that the spacer is not needed. It actually does state this on the box, that some models will have to remove the spacer. However, the spacer looked like part of the cam to me when I was in there, never figured out it was a seperate piece.
So you can all take shots at me for my ineptitude...

 

Glad you got it figured out! It would have been interesting to know if a mechanic's stethescope would have found the sound in the timing cover.
Did it damage the timing cover at all?